carB (PP_4723; UniProt Q88DU6) encodes the large (catalytic) subunit of carbamoyl phosphate synthetase (CPS; EC 6.3.4.16 / 6.3.5.5). Together with the glutaminase small subunit (carA), it catalyzes the ATP-dependent synthesis of carbamoyl phosphate in the cytoplasm from bicarbonate, two molecules of MgATP, and ammonia derived from glutamine:
L-glutamine + 2 ATP + HCO₃⁻ + H₂O → carbamoyl phosphate + L-glutamate + 2 ADP + Pᵢ
Carbamoyl phosphate is the single shared precursor that feeds both the arginine biosynthetic pathway and the pyrimidine nucleotide biosynthetic pathway. The large subunit itself performs the two ATP-dependent phosphorylation steps (bicarbonate → carboxyphosphate; carbamate → carbamoyl phosphate) at two internal ATP‑grasp active sites, and carries the C‑terminal allosteric domain through which the enzyme is feedback‑regulated.
Gene identity is confirmed: the symbol carB, the organism (P. putida KT2440), the CarB family / HAMAP rule MF_01210, and the ATP‑grasp + CPSase domain architecture all match the UniProt target. No gene‑symbol ambiguity was encountered — carB is a universally conserved, well‑characterized enzyme name.
Sequence analysis of Q88DU6 (retrieved from UniProt) confirms a 1,073-residue, ~117 kDa protein with the canonical CPS large-subunit organization:
| Region | Residues | Role |
|---|---|---|
| Carboxyphosphate synthetic domain (ATP‑grasp 1: 133–328) | 1–403 | Phosphorylation of bicarbonate → carboxyphosphate; ammonia attack → carbamate |
| Oligomerization domain | 404–553 | Subunit/oligomer contacts |
| Carbamoyl phosphate synthetic domain (ATP‑grasp 2: 678–869) | 554–935 | Phosphorylation of carbamate → carbamoyl phosphate |
| Allosteric / MGS‑like domain | 936–1073 | Binds allosteric effectors (UMP, IMP, ornithine) |
Domain signatures: ATP‑grasp (IPR011761), CPSase large-subunit (IPR006275), pre‑ATP‑grasp (IPR058047), and MGS‑like (IPR011607; Pfam PF02142); governed by HAMAP rule MF_01210 (CarB family). Numerous nucleotide/substrate binding-site residues are annotated within both ATP‑grasp folds, confirming two independent catalytic sites. This architecture is a one-to-one match to the crystallographically characterized E. coli enzyme, so its detailed mechanism transfers with high confidence.
Ortholog-level confirmation (this work): A global (Needleman–Wunsch) alignment of Q88DU6 against E. coli CarB (P00968) shows 78.9 % sequence identity over the full length (both proteins are exactly 1,073 residues). Experimentally defined E. coli functional residues are conserved in P. putida: the UMP/IMP effector anchor Lys993→Lys992, IMP anchors Lys954→Lys953 and Asn1015→Asn1014, and the ornithine-activator/signal-transmission residues Glu783→Glu782, Glu892→Glu891, Thr1042→Thr1041, plus K‑loop Glu761→Glu760. The one notable exception is the IMP photoaffinity-labeling residue His995, which is a threonine (His995→Thr994) in P. putida — predicting a possibly attenuated/altered IMP activation while UMP inhibition and ornithine activation are retained. This near-identity (same length, ~79 % identity, conserved active-site chemistry) makes transfer of the E. coli mechanism to P. putida essentially certain.
Catalytic-site conservation (this work): Mapping the E. coli catalytic/Mg²⁺-coordinating residues onto Q88DU6 shows 9/9 probed active-site residues are identical — N-terminal carboxyphosphate site (Glu215, Glu299, Asn301, Glu383) and C-terminal carbamoyl-phosphate site (Glu760/K-loop, Glu840, Asn842, Glu915) — demonstrating that both ATP-dependent phosphorylation active sites are structurally and catalytically intact in P. putida. This upgrades the mechanistic transfer from "family-level inference" to "conserved-active-site evidence."
Curated pathway/quaternary annotations (UniProt): Both catalytic activities are annotated — glutamine-dependent (HCO₃⁻ + L‑Gln + 2 ATP → carbamoyl phosphate + L‑Glu + 2 ADP + Pᵢ) and ammonia-dependent (HCO₃⁻ + NH₄⁺ + 2 ATP → carbamoyl phosphate + 2 ADP + Pᵢ). Pathway assignments: L‑arginine biosynthesis — carbamoyl phosphate from bicarbonate, step 1/1; and pyrimidine (UMP) de novo biosynthesis — step 1/3 toward (S)-dihydroorotate. The holoenzyme is a tetramer of heterodimers, (αβ)₄ (KEGG ppu:PP_4723; eggNOG COG0458).
CPS assembles carbamoyl phosphate through a minimum of four consecutive reactions involving three unstable intermediates (ammonia, carboxyphosphate, carbamate) [PMID 9818189, 10387030]:
Substrate specificity / nitrogen source: CPS can use either glutamine (physiological, EC 6.3.5.5) or free ammonia (EC 6.3.4.16) as the nitrogen donor; the large subunit strictly requires ammonia, which in vivo is supplied by the carA glutaminase [PMID 11212301, 10387030]. Bicarbonate (not CO₂) is the carbon/oxygen source, and two MgATP are consumed per product. K⁺ is a required activating monovalent cation.
Evolutionary/structural inference: The large subunit is built as two homologous halves related by a near-exact internal 2‑fold axis, indicating evolution from a homodimeric ancestor; each half's first three domains resemble biotin carboxylase (the ATP‑grasp fold) [PMID 9174345].
CPS is a soluble cytoplasmic biosynthetic enzyme. A defining structural feature is that its three active sites are widely separated (~45 Å apart) and connected by an internal molecular tunnel spanning ~96–100 Å; reactive intermediates travel this tunnel rather than being released to solvent [PMID 9174345, 11212301, 9818189]. This intramolecular channeling protects the chemically labile carboxyphosphate and carbamate. Catalysis at the three sites is kinetically synchronized: glutamine hydrolysis is stimulated ~275‑fold by ATP/bicarbonate turnover, coordinating the small‑subunit and large‑subunit chemistry [PMID 10819970].
Carbamoyl phosphate stands at the crossroad of arginine and pyrimidine biosynthesis; in E. coli and most other Gram‑negative bacteria (including P. putida) it is produced by a single CPSase encoded by the carAB operon [PMID 30238253]. The P. putida KT2440 carB (PP_4723) is the large-subunit gene of this operon.
Operon / partner subunit (this work): In the KT2440 genome, carB (PP_4723) is immediately adjacent to PP_4724 = carA (Q88DU5), the glutaminase small chain (378 aa, EC 6.3.5.5), confirming a single, co-localized carA/carB gene pair that encodes one glutamine-dependent CPS — the Gram-negative paradigm, in contrast to Bacillus (two pathway-specific isozymes) [PMID 12561954]. In vivo, CarB's ammonia substrate is supplied by CarA's hydrolysis of glutamine; free NH₄⁺ can substitute in vitro (accounting for the dual EC 6.3.5.5 / 6.3.4.16 assignment). (The upstream gene PP_4722 is the unrelated greA.)
Because one metabolite serves two pathways, enzyme activity is allosterically tuned by effectors from both consuming pathways plus purine metabolism, all acting through the C‑terminal allosteric domain (conserved as res 936–1073 in Q88DU6):
- UMP (pyrimidine end-product) — inhibitor
- Ornithine (arginine pathway intermediate) — activator
- IMP (purine) — activator
Structural studies localize IMP binding to the C‑terminal β‑sheet domain (Ser937–Lys1073), with Lys993/His995 crosslinked by UMP/IMP in overlapping sites [PMID 10428826, 10194302].
Pseudomonas-specific regulation: In Pseudomonas, the carAB operon (containing carB) is transcribed as a single unit controlled by both arginine and pyrimidines — arginine modulates transcription initiation while pyrimidine control is exerted post-initiation [PMID 8169201]. carA and carB are contiguous in P. stutzeri and in P. putida KT2440 (PP_4724/PP_4723), whereas in P. aeruginosa they are separated by a 216-aa ORF yet co-transcribed [PMID 8169201]. Notably, an early direct study in P. putida found only modest (≤5-fold) repression/derepression of arginine and pyrimidine biosynthetic enzymes [PMID 176312], implying that flux through carbamoyl phosphate is governed substantially by allosteric modulation of CPS activity in addition to this relatively loose transcriptional control. Effector binding also modulates oligomeric state: ornithine and IMP favor an (αβ)₄ tetramer, whereas UMP favors an (αβ)₂ dimer [PMID 11551199]. Mutagenesis has mapped the ornithine signal-transmission conduit (E783, E892, T1042, K‑loop residues) linking the effector site to the carbamate‑phosphorylation active site [PMID 11913967, 11943174].
Supported:
- H1: carB is the CPS large (synthetase) subunit catalyzing carbamoyl phosphate formation — supported (annotation + orthology + domain match).
- H2: The large subunit performs two ATP‑dependent phosphorylations at two ATP‑grasp sites — supported (structure + kinetics; confirmed in Q88DU6 sequence).
- H3: Product feeds arginine + pyrimidine pathways; enzyme is allosterically regulated (UMP/ornithine/IMP) via a conserved C‑terminal domain — supported.
- H4: Function is cytoplasmic with internal substrate channeling — supported.
Refuted / not applicable:
- Gene-symbol ambiguity concern — refuted; carB unambiguously denotes CPS large chain across bacteria.